feat: allow usage of custom keypair in stateful convenience methods

[marc0olo]

closes #230

[codecov-commenter]

Codecov Report

Merging #231 (871b098) into 3.x (c03ca1c) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##                3.x     #231      +/-   ##
============================================
- Coverage     11.27%   11.25%   -0.03%     
  Complexity      150      150              
============================================
  Files           152      152              
  Lines          3743     3750       +7     
  Branches        139      141       +2     
============================================
  Hits            422      422              
- Misses         3295     3302       +7     
  Partials         26       26              

Impacted Files	Coverage Δ
.../service/transaction/domain/ContractTxOptions.java	0.00% <0.00%> (ø)
...rvice/transaction/impl/TransactionServiceImpl.java	0.00% <0.00%> (ø)

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[mitch-lbw]

lgtm